The uniformity of abundance gradients as a function of scale length
suggests a close correlation between metallicity and disk surface
brightness. Indeed,
McCall (1982) and
Edmunds & Pagel
(1984)
noted a remarkably tight correlation between O/H and disk surface
brightness for late-type spirals. This has provided part of the
motivation for
models of self-regulated star formation, in which the radiation and
mechanical energy produced by stars feeds back into the surrounding
ISM and acts to inhibit further star formation. Models of this kind
have been explored by
Phillips & Edmunds
(1991)
and Ryder (1995),
and appear to do a good job of reproducing the trends of both star
formation rate and O/H with surface brightness. One caveat is that
the interaction of the stellar energy output with the ISM is still
poorly understood. Viscous disk models also provide a possible mechanism
to tie the abundances to the underlying surface density distribution.

Edmunds & Pagel
(1984)
also noted that early-type spirals do not follow
the same O/H-surface brightness correlation as the late types.
Garnett et al. (1997a)
put this on a more quantitative basis. Figure 13
displays the characteristic metallicity at two fixed
values of disk surface brightness for a sample of spirals having either
I- or R-band surface photometry; these bandpasses presumably sample the
light from the old disk population better than B. The figure shows that
metallicity-luminosity correlation appears to hold at all values of
surface brightness across spiral disks. This result argues for two
modes of enrichment in disk galaxies: a local mode, in which the
metallicity is connected to the local mass density, and a global
mode, in which an entire galaxy is enriched in a manner dependent
on its total mass. One can imagine a global enrichment event which
raises the metallicity of a galaxy to some level which depends on
total mass, followed by sequential local enrichment which follows
the mass density distribution. One caveat is that M/L, and
thus the mass surface density at a given surface brightness, may vary
systematically along the luminosity sequence in
Figure 13. A more
comprehensive study of mass surface density and gas fraction along
this sequence should prove enlightening.